Conventionally known thermal transfer image-receiving sheets include natural paper with or without a recording layer formed on its surface. They are poor in surface smoothness. Thus, a recording layer is formed on a thin polypropylene synthetic paper adhered to a natural paper or a thick polypropylene synthetic paper, for an improved smoothness of an image-receiving sheet. This is because polypropylene synthetic paper has, in addition to an adequate cushioning property, surface smoothness that cannot be achieved by natural paper. The adequate cushioning property enables uniform and sufficient contact between heating head/transcription ribbon/image-receiving sheet during thermal transfer, thus resulting in uniform and highly dense printing. When a polypropylene synthetic paper is used as a substrate, however, the image-receiving sheet is susceptible to plastic deformation, and poor flexibility thereof produces wrinkles on its surface even when the image-receiving paper is only slightly bent. This profoundly impairs the grade of the printed matter.
In addition, a method using a porous polyester film instead of polypropylene synthetic paper has been proposed. Nevertheless, a porous polyester film generally has a greater rigidity and less cushioning property than does polypropylene synthetic paper. To achieve the same image density as polypropylene synthetic paper with a porous polyester film, the porosity needs to be made higher than that of polypropylene synthetic paper. Consequently, void size becomes markedly greater to impair surface smoothness or produce wrinkles on the surface. When the porosity becomes greater, moreover, polyester becomes unstable during production, thereby making stable production at industrial scale extremely difficult.
The currently most effective method to obtain a film having functions similar to those of natural paper, from a polyester resin as a main starting material, is as follows. That is, a multitude of fine voids are formed inside a polyester resin film. The voids give adequate flexibility to the film, reduce weight of the film and afford superior writability and vivid printing/transcription effect. The voids are formed in the film by mixing a resin incompatible with the polyester resin with a staring material polyester resin as a void-forming agent, processing the mixture to give a film and stretching the film to create voids in the interface between resins.
As the void-forming agent, proposed are polyolefin resin such as polypropylene resin and polymethylpentene resin (Japanese Patent Unexamined Publication No. 34755/1974), polystyrene resin (Japanese Patent Examined Publication Nos. 2016/1974 and 29550/1949) and the like.
Of these, polyolefin resin, particularly polymethylpentene, has superior void-forming capability and is very superior in reducing the weight of the film. On the other hand, it has poor compatibility with polyester, and said void-forming agent is dispersed as crude particles in polyester. Consequently, the size of the void also becomes greater and the greater void size combined with the nerve of the polyester causes vastly decreased flexibility of the film. This problem becomes prominent during the production and processing of the film or handling of the final product such as a printed matter. For example, when the film is slightly bent, it is easily folded or writes are developed on the film surface. Flexibility is one of the most advantageous properties of synthetic resin film, and the loss of this property is fatal for the film.
On the other hand, polystyrene resin can be used to form a number of fine voids because it has a low degree of incompatibility with polyester resin as compared to polyolefin resin. The low degree of incompatibility allows fine dispersion of a void-forming agent. This in tun makes it possible to minimize degradation of flexibility of the film which has been a fatal defect of polyolefin resin. However, the void-forming agent tends to collapse in the direction of film thickness during stretching of the film. Consequently, sufficient growth of voids is prevented and the film cannot be lightweight.
Some methods have been proposed to maintain superior void-forming capability of the polyolefin resin and to improve dispersion of the agents in the polyester. For example, Japanese Patent Examined Publication No. 17779/1995 proposes addition of surfactant to polyester, Japanese Patent Unexamined Publication No. 235942/1990 proposes addition of polyethylene glycol and Japanese Patent Unexamined Publication No. 264141/1992 proposes addition of polyether ester copolymer.
Fine dispersion of polyolefin resin by these methods can be achieved only to a certain extent and fine dispersion as achieved by polystyrene resin is nearly impossible to achieve. Stable fine dispersion cannot be afforded without damage to other properties such as strength of the film and whiteness thereof.
In other words, a method based on the addition of surfactant is associated with inevitable denaturing that occurs in a melt line due to insufficient heat resistance of the surfactant, and stable dispersion effect cannot be achieved.
The method comprising addition of a polyether resin is associated with decomposition of polyether during drying or in melt line of the resin or recycling of film waste, since polyether is extremely susceptible to degradation by heat. The decomposition of polyether not only brings about inconsistent fine dispersion of polyolefin resin, but also poses problems. The problems include, for example, lower molecular weight of polyester and thereby caused lower film strength, yellowing of the film, pungent smell due to the generation of aldehyde and the like.
Thus, a heat-resistant porous polyester film having both superior void-forming capability of polyolefin resin, particularly polymethylpentene, and superior flexibility of polystyrene resin is not in existence in the prior art technology.